The present invention relates to a over-sampling modulator that has been widely used as the A/D converter and the D/A converter for audio band. More particularly, this invention relates to the modulator with a feed-forward configuration that can improve the frequency characteristics of a signal transfer function.
A conventional modulator will be explained first. At present, the over-sampling delta-sigma conversion system has been widely used in the A/D or D/A converters for the audio band. FIG. 13 is a diagram that shows a configuration of the modulator with a conventional feed-forward configuration that employs the over-sampling delta-sigma conversion system. The reference number 101 denotes an attenuator block (b), 102 and 105 denote adders (Add), 103 and 104 denote integrators (Int), 106 denotes a quantizer (Q), and 107 denotes a delay element (zxe2x88x921).
The attenuator block 101 is the first component provided at the signal entry side of the modulator to prevent an input of over-amplitude. In other words, when the modulator becomes in a higher order configuration, number of stages of integrators increases, and the amplitude becomes large at the final stage. Therefore, the attenuator block is essential in order to prevent oscillation.
Operation of the conventional modulator will now be explained. According to the feed-forward system, the adder 102 subtracts a quantized signal that has a delay of 1 sample period, from a signal output from the attenuator block 101. The two-stage integrators integrate a result of this subtraction, and the adder 105 adds respective outputs of these two-stage integrators. The quantizer 106 quantizes a result of this addition. A result of the quantization (an input signal component+ quantizing noise) is fed back to the adder 102 via the delay element 107.
This conventional modulator modulates quantizing noise according to the order of the modulator. In other words, the modulator improves SNR (Signal to Noise Ratio) characteristic by concentrating a frequency distribution of the quantizing noise to a high area, and by minimizing the influence of noise in a low-frequency area (refer to FIG. 14).
However, according to the conventional modulator, the frequency characteristics of a signal transfer function (STF) have an inclination as shown by the following equation (1), and a peaking or attenuation may occur in a high area.                               Y          =                      xe2x80x83                    ⁢                                                    {                                  bX                  -                                                            z                                              -                        1                                                              ⁢                    Y                                                  }                            ⁢                              1                                  1                  -                                      z                                          -                      1                                                                                  ⁢                              {                                  1                  +                                      1                                          1                      -                                              z                                                  -                          1                                                                                                                    }                                      +            q                          ⁢                  
                ⁢                              Y            ⁡                          [                                                                    (                                          1                      -                                              z                                                  -                          1                                                                                      )                                    2                                +                                                      z                                          -                      1                                                        ⁢                                      {                                                                  (                                                  1                          -                                                      z                                                          -                              1                                                                                                      )                                            +                      1                                        }                                                              ]                                =                      xe2x80x83                    ⁢                                    bX              ⁢                              {                                                      (                                          1                      -                                              z                                                  -                          1                                                                                      )                                    +                  1                                }                                      +                                                            (                                      1                    -                                          z                                              -                        1                                                                              )                                2                            ⁢              q                                      ⁢                  
                ⁢                  Y          =                      bX            +                                          b                ⁡                                  (                                      1                    -                                          z                                              -                        1                                                                              )                                            ⁢              X                        +                                                            (                                      1                    -                                          z                                              -                        1                                                                              )                                2                            ⁢              q                                                          (        1        )            
In the equation (1), bX+b (1xe2x88x92zxe2x88x921) X expresses the above frequency characteristics.
Therefore, according to the conventional modulator, there has been a problem that it is necessary to provide a correction filter before or after the modulator, in order to correct the frequency characteristics within a signal area when the signal area extends to a higher frequency band.
It is an object of the invention to provide a modulator that has a feed-forward configuration which can hold the frequency characteristics of a signal transfer function at a constant level, based on an insertion of a simple circuit block.
The modulator according to the present invention employs an over-sampling delta-sigma conversion system. The modulator comprises an attenuating unit which attenuates an input signal to produce an attenuated signal, a delay unit which gives a delay of 1 sample period to the attenuated signal to produce a delayed signal, a subtracting unit which subtracts a quantized signal that has been fed back with a delay of 1 sample period from the delayed signal to produce a subtracted signal, integrating units provided in n-stages, where n is an integer is equal to or larger than 2, which integrate the subtracted signal to produce integrated signal, an adding unit which adds the integrated signal output from each integrating unit and the attenuated signal to produce an added signal, and a quantizing unit which quantizes the added signal to produce an output signal, outputs the output signal and feeds back the output signal to the subtracting unit.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.